Ink jet recording is a preferred recording method for use in terminal printers and the like for its advantages such as being noiseless and suitability for high speed recording; it has come into wide use in recent years. In addition, multicolor recording can be effected with ease by using multiple ink nozzles therein. Therefore, various ink jet recording processes have been examined for multicolor recording application.
In particular, because ink jet printers can draw complex images rapidly and exactly, utilization of ink jet printers as apparatus for forming hard copies of the image information including characters and a wide variety of figures drawn with electronic computers has recently been watched with keen interest. Further, there is an increasing need of recording the image information drawn with an electronic computer on transparent recording sheets by means of an ink jet printer, the thus obtained prints being utilized as originals for OHP or the like.
Also, ink recording sheets are employed for plotters and handwriting using aqueous ink pens instead of an ink jet.
As for the ink for ink recording, aqueous ink containing water as a main component is used to advantage in respect of safety and recording characteristics. In many cases, polyhydric alcohols or the like are added to the aqueous ink for the purposes of preventing a pen or a nozzle from choking and enhancing the jetting characteristics.
In those ink recording methods, ordinary paper and so-called ink recording paper having a porous ink-absorbing layer provided on a support have so far been used as a recording sheet. However, these conventional recording sheets have some problems when used as originals for the above-described OHP. For instance, they are, in general, poor in photo transmissibility, or interior in ink acceptability if they have sufficient transparency. More specifically, in many conventional recording sheets, a method of forming a porous ink-absorbing layer on the surface, and causing the sheet to absorb ink in voids of the porous layer and to fix it firmly therein is employed. Accordingly, even when a transparent support is used, there is the problem that the porosity of the ink-absorbing layer aggravates the photo transmissibility.
On the other hand, when the surface of the ink-absorbing layer is nonporous, there is the different problem that since the recording sheet is inferior in aqueous ink acceptability though it has improved photo transmissibility, the aqueous ink remains on the sheet surface for a long time after image recording, thus prolonging the dry fixation time.
For the purpose of solving the foregoing problems, there have been proposed a number of recording sheets which have on a transparent support, a transparent ink-absorbing layer having a high degree of aqueous ink acceptability. Examples of materials to constitute such an ink-absorbing layer include the combined use of polyvinyl alcohol and a water-soluble high molecular weight compound of a polyacrylic acid type as disclosed in JP-A-60-168651 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"), the use of hydroxyethyl cellulose as in JP-A-60-262685, the use of a mixture of carboxymethyl cellulose and polyethylene oxide as in JP-A-61-181679, the use of a mixture of a water-soluble cellulose and polyvinylpyrrolidone as in JP-A-61-193879, and the use of a polyalkylene oxide crosslinked by an isocyanate compound as in JP-A-61-32787.
The recording sheets disclosed in these patents, though they have advantages in that they are excellent in photo transmissibility and high in ink acceptability, have some of the following problems:
(1) The ink absorptivity just after image recording is still insufficient, thereby causing unevenness in image density (particularly in the uniformly drawn portions).
That is, fine droplets of aqueous ink are not absorbed by the ink-absorbing layer at the instant of adhering to the surface of the recording sheet; instead it takes the recording sheet from several seconds to several minutes to absorb the major part of the ink droplets attached. In this period, the fine droplets are united into large droplets, so the portions intended to look uniform (uniformly drawn portion) seem to have an uneven or mottled pattern. To make the matter worse, the unevenness or mottled pattern becomes particularly conspicuous when enlarged on a screen by projection with OHP.
(2) In recording images on sheets, each sheet is conveyed with rollers. Accordingly, when the ink absorptivity just after image recording is insufficient, the sheet and image suffer from roller marks.
(3) Since the aqueous ink shows little diffusibility in the lateral direction of the ink absorbing layer when attached to the image recording sheet, there are vacancies and noncovered area between the ink droplets, thus lowering the apparent optical density.
(4) When put on OHP, the recording sheet curls significantly.